1. Field of the Invention
The invention relates to color measurements and more particularly to a scanning colorimeter in which a movable head collects color information from an area along the path of travel of the head.
2. Background Art
Colorimeters, such as densitometers and spectrophotometers, are well known in the color measuring art. Densitometers are widely used, for example, as part of quality control of colors in the printing industry, to obtain measurements of color which are compared against a predetermined standard. Spectrophotometers are commonly used to identify colors for color matching purposes. It is common practice in the printing industry to provide color bars along one side of a sheet of paper. Scanning densitometers are commercially available which measure the various color bars and provide color information to a processing unit for further analysis. Typically, the scanning densitometers include a measuring head supported on a bridge or rails and moved along the color bars to measure color density. In typical prior art densitometers, the scanning heads are continuously in electrical communication with a stationary host computer so as to transmit parameter data and color measurement data between the scanning head and the computer system. The electrical cabling required for such communications can be of substantial weight. Several known densitometers require relatively heavy duty motors and control arrangements for moving the head. One of the difficulties in scanning densitometers is the accurate positioning of the head with its optics relative to the edges of the color bars. As the number of color bars is increased, the widths of the color bars and separation between adjacent color bars tend to be reduced and require a greater accuracy of positioning is required.
In the printing industry, it is often desirable to use two adjacent rows of color bars near one edge of a printed paper. Prior art scanning densitometers require that the paper be separately positioned with respect to the densitometer for each of the rows of color bars. It is, therefore, desirable to provide a scanning densitometer capable of reading color bars of adjacent rows without a repositioning of the paper.
A scanning densitometer preferably includes a paper stop such that a sheet may be conveniently placed against the stop without concern for special alignment of a row of color bars with respect to the path of travel of the densitometer head. However, in some applications, the color bar is immediately adjacent the edge of the paper and when the paper with the color bar immediately adjacent the edge is placed against the paper stop, the paper stop may interfere with the optical head, thereby preventing proper color measurements. Hence, other measures must be taken to assure proper positioning of the color bars relative to the path of the head.
Principles of densitometry are well known. A prior art densitometer typically includes a light source which projects light onto the object sampled to be measured in a direction which is normal to the plane of the object sampled. Light reflected from the object sampled is detected by means of three separate filters, each disposed at an angle of 45.degree. from normal. The three separate filters are employed for purposes of discriminating the read, green, and blue spectral responses sensors associated with each of the filters detect white rays emanating through the corresponding filters and provide and electrical output signal proportional to the intensity of the detected light rays. Electrical signals generated by the detectors are supplied to signal processing circuitry which computes data representative of the color measurements.
A prior art scanning densitometer is disclosed in U.S. Pat. No. 5,073,028, issued Dec. 17, 1991. That scanning densitometer includes an autonomous densitometer head including a motor propelling the densitometer head along transport bars which may be raised and lowered to insert a sample sheet under the transport bars. The prior art densitometer includes a docking end provided with an optical coupler and electrical coupler. A corresponding optical coupler and electrical coupler are provided in the autonomously moving head. Densitometer measurements are obtained by the head as it travels over color bars or the like to be measured, storing color information in a memory internal to the head. When the measurement is completed, the unit returns to the docking end and transfers data via the optical coupler and connectors on the docking end to a host computer. While at the docking end, electrical connection is made to the head to charge an internal battery which drives the internal motor as well as information processing and storing circuitry. A disadvantage of that system, however, is that the transport arrangement is relatively expensive and accurate alignment of color bars and the like with the path of travel of the head is required.
A disadvantage of prior scanning densitometers is that the densitometer head tends to be relatively bulky and heavy and it is often difficult to rapidly move the head and obtain accurate positioning. A further disadvantage of densitometers is that a limited amount of information regarding the reflected light is obtained by the standard three filters and associated detectors of a standard densitometer.